A large proportion of the pollution of the atmosphere by the exhaust fumes of conventional automotive engines is due to the incomplete combustion of gasoline or other hydrocarbon fuels. In operation, an airstream aspirated through a Venturi throat at the inlet of a duct within the carburetor has a velocity determined, on the one hand, by the setting of an accelerator-controlled butterfly valve in the duct and, on the other hand, by the suction developed in the combustion chamber or chambers. Upon the release of the depressed accelerator pedal by the driver, the duct is progressively throttled by the butterfly valve whereby, for a given engine speed, the flow velocity of the airstream and hence the Venturi effect gradually increase so that relatively more fuel is drawn in. Thus the valve should be so designed that during normal driving, with engine speed varying roughly in proportion to the free cross-section of the duct, the fuel/air ratio in the explosive mixture reaching the cylinders is adapted to the load to insure a reasonably clean combustion. If, however, the engine runs at an above-normal speed with the butterfly valve in a nearly closed limiting position, as when the engine is used for deceleration, the mixture will be too rich so that combustion will be incomplete. A similar situation exists when the engine is idling, the mixture then generally containing too much fuel for the small thrust required. Providing a bypass for additional air is not a satisfactory solution since then the mixture becomes too lean under other driving conditions. Complex and correspondingly expensive systems, including catalytic afterburners, have therefore been devised for the purpose of properly dosing the fuel supply.
Another parameter affecting the air/fuel ratio, to which little attention has been paid heretofore in this context, is the pressure under which the fuel is delivered to the injection nozzle. Unless this pressure is held substantially constant, the supply rate will be subject to variations unrelated to engine speed and accelerator position. A conventional float-controlled pressure regulator, inserted between the engine-driven fuel pump and the carburetor, does not fully solve this problem inasmuch as a spurt in engine speed can still elevate the instantaneous pump pressure to a value overcoming the force with which the needle valve of the regulator is urged into its blocking position by the buoyancy of the float.